This invention relates to an engine control system and more particularly to an improved arrangement for controlling the timing of an electrically controlled event in an internal combustion engine.
A number of events in an internal combustion engine, particularly a spark ignited engine, are controlled to occur at a particular angular relationship of the engine output shaft. The angular output shaft position is important because it is related to the particular instantaneous volume of the combustion chamber wherein the event occurs. Such events may be either the timing of the firing of a spark plug, or the control of fuel injection, either the starting, ending or duration of such fuel injection.
Although the events should be timed in relation to engine output shaft angle, the existing methods for initiating the timing also use an actual or real time variant in addition to angular output shaft position. The reason for this is that it is very difficult and expensive to measure each incremental rotatable angle of the engine output shaft. Therefore, one of two basic methods have been employed for determining the timing of events in an engine.
The first method of determining the timing is to use a reference output shaft angle as a starting point for measuring the timing. The time for the engine output shaft to rotate from this reference angle position to a position at which the event is to occur is based upon the average rotational speed of the output shaft and the event is initiated at a time interval after the reference crank angle position passes determined by this average rotational speed. However, as is well known, the speed of rotation of the output shaft of an engine varies not only from revolution to revolution but during a single revolution. Therefore, the average speed calculation afore referred to does not provide an accurate indication of output shaft angle under all circumstances and represents at best an approximation.
Another method of timing the event is to employ a toothed wheel that is driven with the engine output shaft and a sensor that sends out pulses for each incremental rotation of the toothed wheel. This type of system can offer greater accuracy, however, has its own disadvantages. In the first instance, the sensitivity of the device will be dependent upon the number of teeth employed. However, if the number of teeth employed is increased to increase the accuracy, then not only the cost of the system become elevated, but extraneous noise becomes a problem. That is, complicated electrical circuities are required to discriminate between the passing of the teeth on the wheel and extraneous noise. Furthermore, the timing of the event does not always occur at an exact interval when a tooth will pass the sensor. Hence, it is still necessary to make some time calculation to determine incremental rotation between the output pulses of the teeth.
A system has been provided to improve the accuracy of such timing and is disclosed in the co-pending application entitled "Engine Control System", filed in the name of Hitoshi Motose and Akihiko Hoshiba, Ser. No. 07/822,175, filed Jan. 16, 1992, and assigned to the Assignee hereof. However, the system disclosed in that application is also dependent upon a toothed wheel and sensors that provide a reference shaft position as well as incremental shaft angular position.
In addition to the problems as aforenoted, when two sensors are employed there is a difficulty in insuring the accurate angular location of one sensor relative to the other. This problem is even further compounded when each sensor cooperates with a respective toothed wheel. The toothed wheels may have different angular positions relative to the output shaft. These misalignments between sensors and or toothed wheels can result in further inaccuracies, regardless of how sophisticated the control routine may be.
It is, therefore, a principal object to this invention to provide an improved arrangement for controlling the timing of an event of an internal combustion engine.
It is a further object to this invention to provide an improved arrangement for controlling the timing of the event of an engine that employs two sensors and which can compensate for misalignment between the sensors.